The present disclosure relates to a reading module that is used in digital copiers, image scanners, etc., and that irradiates a document with light and reads image light reflected from the document, and the present disclosure also relates to an image reading device and an image forming apparatus provided with such a reading module.
Conventional reading methods that have been adopted in image reading devices incorporated in copiers and the like employing the electro-photographic process include the CCD method, in which a charge-coupled device called a CCD sensor is used, and the CIS method, in which a photoelectric conversion element called a CMOS(Complementary MOS) sensor is used.
In the CCD method, image reading is performed by imaging a reduced image of a document, on an image sensor having a size that is one-fifth to one-ninth as large as that of a document, by using a plurality of flat mirrors and an optical lens. Merits of the CCD method include an advantage of a deep depth of field. Here, the depth of field is the range in which, even when a subject (here a document) is displaced in the direction of the optical axis from the in-focus position, the subject can be seen as if in focus. This means that, with a deep depth of field, even when the document is displaced from a normal position, it is possible to obtain an image comparable to an image that would be obtained when the document is located at the normal position.
On the other hand, demerits of the CCD method include a very long optical path length (which is a distance over which light travels from a subject to the sensor), which is 200 to 500 mm. In image reading devices, for the purpose of securing the optical path length in a limited space in a carriage, a plurality of plane mirrors are used to change the direction in which light travels. Thus, image reading devices incorporate a large number of components, which results in high cost. Further, in a case where a lens is used in an optical system, chromatic aberration is caused by variation in refractive index with wavelength. To correct such chromatic aberration, it is necessary to provide a plurality of lenses. Using a plurality of lenses in this manner also results in high cost.
In the CIS method, image reading is performed by imaging an image, on an image sensor having a size similar to that of a document, by using a plurality of erect-image rod-lenses with unity magnification arranged in an array. Merits of the CIS method include a comparatively short optical path, which is 10 mm to 20 mm, which is shorter than that of the CCD method, and thus helps achieve compact image reading devices. Furthermore, imaging by use of rod lenses alone eliminates the need for mirrors, which are required for imaging in the CCD method, and this helps achieve a thin and simple-structured scanner unit incorporating a CIS sensor, and the simple structure contributes to cost reduction. On the other hand, in the CIS method, the depth of field is so small that, when the document is displaced from the normal position in the optical axis direction, a severe blur results from image bleeding due to magnification difference between the individual lenses. This results in a demerit of the CIS method, that is, incapability of uniformly reading a book document or a document with an uneven surface.
In recent years, a method has been proposed which is different from the CCD method and the CIS method described above in that image reading is performed by employing a reflection mirror array in an imaging optical system. In this method, a plurality of reflection mirrors are arranged in an array, and a document read in different reading regions corresponding to the reflection mirrors on a region-by-region basis is formed into an inverted reduced image on a sensor. However, unlike in the CIS method where a rod-lens array is used, one region is read and imaged with one optical system. Further, by adopting a telecentric optical system in the imaging method, in reading a plurality of regions of a document on a region-to region basis, there occurs no image bleeding, which would otherwise be caused due to overlapping of images of different magnifications with each other, and thus image blurring is reduced and a compound-eye reading method is achieved.
Furthermore, unlike in a case where a lens is used in the optical system, this method, where the optical system is constituted by mirrors alone, suffers no chromatic aberration. Thus, there is no need of chromatic aberration correction, and this helps reduce the number of elements constituting the optical system.